The widespread and increasing use of fiber optics for communication purposes has brought considerable attention to means for increasing the communication capacity of the fibers. The use of Wavelength Division Multiplexing (WDM) signals is an effective way of increasing transmission capacity, since a plurality of separated wavelength bands, each band capable of transmitting signals, is simultaneously used in a fiber.
Optical amplifiers are used to compensate for the fiber link and splitting losses within optical communication systems. The WDM technique requires amplification in a broad wavelength spectrum, which is typically realized by letting a plurality of optical amplifiers, each operational in a part of the wavelength spectrum, operate in parallel. Various rare-earth doped optical amplifiers, in the form of amplification fibers, suitable for WDM are known in the art. The optical fiber amplifier typically consists of an optical material such as glass, combined with a rare earth dopant and configured as an optical waveguide. Rare-earth-doped silica fibers are popular today, in part because they offer the advantages of single-mode guided wave optics. Optical fiber amplifiers can be made to operate over a broad range of wavelengths, dictated by the atomic properties of the host and rare earth dopant. In their basic operation pump light from a pump source is used to excite the dopant atoms in the amplification fiber from a ground state to a metastable state. The excited atoms are, by an incident signal of a characteristic wavelength, stimulated to emit energy in form of a light of the same wavelength, the emission resulting in an amplification of the signal. The process is known as stimulated emission. After the stimulated emission the atom is in a termination state. The gain should be high, the efficiency too. The gain of the amplifier, defined as the power of the signal out divided by the power of the signal in, should be as high as possible. Also, the efficiency of the amplifier, measured as the gain achieved for a particular pump power, should be as high as possible. In addition the gain should preferably be essentially flat, i.e. not depending on the frequency of the incident light, over the frequency range in which the amplifier is intended to be used.
For amplification in the so called S-band (1450–1520 nm) much interest has been given to thulium doped fiber amplifiers (TDFA). The useful transition for the stimulated emission is between the metastable 3H4 level to the lower 3F4 termination level, giving an output signal of wavelength around 1470 nm. A well recognized problem with using TDFA as an optical amplifier is that the 3H4 level has a short lifetime compared to the 3F4 level. In practice this results in that, even if the pumping excitation to the 3H4 level is successfully achieved, the stimulated emission will be impaired due to the accumulation of population at the 3F4 level. This results in a poor gain.
Several attempts for increasing the gain of the TDFA have been reported, including up-conversion pumping with repeated pumping with the same wavelength and various dual wavelength pumping schemes as for example presented in the following: F. Roy, D. Bayart, A. Le Sauze and P. Baniel, “Noise and gain band management of thulium-doped fiber amplifier with dual-wavelength pumping schemes,” IEEE Photon. Technol. Lett ., 13, 788–790, (2001); T. Kasamatsu, Y. Yano and H. Sekita, “1.50-μm-band gain-shifted thulium-doped fiber amplifier with 1.05- and 1.56-μm dual-wavelength pumping,” Opt. Lett ., 24, 1684–1686, (1999); T. Kasamatsu, Y. Yano and T. Ono, “Laser-diode pumping (1.4 and 1.56 μm) of gain-shifted thulium-doped fiber amplifier,” Electron. Lett ., 36, 1607–1609, (2000); T. Kasamatsu, Y. Yano, and T. Ono, “Gain-shifted dual-wavelength-pumped thulium-doped fiber amplifier for WDM signals in the 1.48–1.51-μm wavelength region,” IEEE Photon. Technol. Lett ., 13, 31–33, 2001; F Roy, F. Leplingard, L. Lorcy, A. Le Sauze, P Baniel,. D. Bayart, “48% power conversion efficiency in single pump gain-shifted thulium-doped fibre amplifier”. Electronics Letters , 37:15, 943–945, 2001; T. Kasamatsu, Y. Yano, T. Ono, “Laser-diode-pumped highly efficient gain-shifted thulium-doped fiber amplifier operating in the 1480–1510-nm band”, IEEE Photonics Technology Letters , 13:5, 433–435, 2001; Tadashi Sakamoto, “S-band fiber optic amplifiers”, Optical Fiber Communication Conference and Exhibit, 2 TuQ1-1–TuQ1-4, 2001; B. Cole, M. L. Dennis, “S-band amplification in a thulium doped silicate fiber” Optical Fiber Communication Conference and Exhibit, 2, TuQ3-1–TuQ3-3, 2001; T. Kasamatsu, Y. Yano, T. Ono, “Laser-diode-pumped highly-efficient gain-shifted thulium doped fiber amplifier operating in the 1480–1510-nm band” Optical Fiber Communication Conference and Exhibit, 2, TuQ4-1–TuQ4-3, 2001; F. Roy, P. Baniel, C. Fages, J. J. Girard, A. Le Sauze, D. Bayart, “Optimal pumping schemes for gain-band management of thulium-doped fiber amplifiers”, Optical Fiber Communication Conference and Exhibit, 2, TuQ7-1–TuQ7-4, 2001; F. Roy, F. Leplingard, L. Lorcy, A. Le Sauze, P. Baniel, D. Bayart, “48% power conversion efficiency in a single-pump gain-shifted thulium-doped fiber amplifier”, Optical Fiber Communication Conference and Exhibit, PD2—1–PD2—3, 2001; F. Roy, D. Bayart, P. Baniel, and “Novel pumping schemes for thulium doped fiber amplifier” Optical Fiber Communication Conference, 2, 14–16, 2000.
In the dual wavelength pumping process, the first photon populates the lower 3F4 level whereas the second photon is responsible for populating the higher 3H4 level and at the same time depopulating the 3F4 (lower) level to allow for population inversion. The reported wavelengths have been 1050–1550 nm for the first pumping and around 1400 for the second pumping and typically a total pump power of 200–500 mW are needed to reach a 27 dB gain.
Disclosed in U.S. Pat. No. 6,407,853 is a dual pumping scheme suitable for TDFA. A first pumping wavelength of preferably 800 nm excite the dopant from the ground level (3H6) directly to the 3H4 level. A second pumping wavelength of 1440 nm is used to depopulate the 3F4 level and hence facilitating the stimulated emission.
The prior art represent significant improvements in the use of thulium doped fiber amplifiers. However, for widespread use in communication systems it is necessary to further increase the efficiency of the amplifiers as well as to use pump wavelengths which are achievable with low cost semiconductor lasers.